Method of forming amino acid-derived diaminopropanols useful as chemical intermediates for protease-inhibitors

ABSTRACT

The present invention relates to a method of forming a 1,3-diamino-3-substituted-2-propanol chemical intermediate from which various chemicals, such as selected protease-inhibitors and other drugs, as well as polymers, can be synthesized. 
     This method includes contacting a nitromethyl amino acid compound with at least one reducing agent to form the 1,3-diamino-3-substituted-2-propanol chemical intermediate.

RELATED APPLICATIONS

This application is a Continuation-in-Part of U.S. application Ser. No.08/271,619, filed Jul. 7, 1994, now U.S. Pat. No. 5,475,138, which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The inhibition of various proteases has application in treating manymedical conditions, such as Alzheimer's disease, retroviral infections,hypotension and hypertension. Many protease-inhibitor compounds havebeen identified. However, the methods for synthesizing theseprotease-inhibitor compounds are often complex and/or expensive.Consequently, methods are needed to produce protease-inhibitor compoundsthrough simpler and/or less expensive processes.

SUMMARY OF THE INVENTION

The present invention relates to a method of forming a1,3-diamino-3-substituted-2-propanol chemical intermediate representedby the following structural formula (structural formula I): ##STR1## orsalts thereof, wherein R¹ is an amino protecting group and R² isselected from the group consisting of --H, C1-C18 alkyl, aryl, acetyland tosyl.

Additionally, R³ is the side-chain of an amino acid such as in an aminoacid, typically having the structure ##STR2## In addition, wherein theamino acid has a chiral center, the side-chain may be from either the Dor L isomer of the amino acid. Further, the amino acid may optionally besubstituted with one or more substitutents, such as halogen, hydroxyl,sulfonate, C1-C3 alkyl, C1-C3 alkoxy and acyl.

Further, R⁴ and R⁵ are each independently selected from the groupconsisting of --H, alkyl, aryl, nitrile and alkoxycarbonyl. However, itis preferred that R⁴ and R⁵ are not both alkoxycarbonyl groups. Inaddition, each R⁶ is independently selected from the group consisting ofhydrogen, lower alkyl, halo-substituted lower alkyl, aryl, alkoxyalkyl,alkoxyaryl.

This method includes contacting a nitromethyl amino acid compound withat least one reducing agent to form said chemical intermediate. Asuitable nitromethyl amino acid compound is represented by the followingstructural formula (structural formula II): ##STR3## wherein R⁶ iseither ##STR4##

The benefits of this invention include the ability to produceprotease-inhibitor compounds, and other drugs, through simpler and/orless expensive synthetic processes.

DETAILED DESCRIPTION OF THE INVENTION

A chemical intermediate, as defined herein comprises a compound fromwhich various chemicals, such as selected protease-inhibitors and otherdrugs, as well as polymers, can be synthesized. In a preferredembodiment, the 1,3-diamino-3-substituted-2-propanol chemicalintermediate is derived from phenylalanine and comprises a1,3-diamino-3-benzyl-2-propanol compound.

Suitable amino protecting groups include protecting groups whichgenerally prevent substitution or addition reactions from occurring witha protected amino group while producing said chemical intermediateaccording to the method of this invention. Examples of suitableprotecting groups include benzyl, t-butyloxycarbonyl (Boc),benzyloxycarbonyl (Cbz), 9-fluorenylmethoxycarbonyl (fmoc),2,2,2-trichloroethoxycarbonyl, 2-haloethoxycarbonyl, benzoyl,phthalimidyl, diphenylphosphinyl and benzenesulfonyl. Alternatively, R¹and R² can be combined to form a protecting group, such as dibenzyl.

Alkyl groups of the present invention include straight-chained, branchedand cyclic alkyl radicals containing up to about 18 carbons. Suitablealkyl groups may be saturated or unsaturated. Further, an alkyl groupmay also be substituted one or more times on one or more carbons withsubstitutents selected form the group consisting of C1-C6 alkyl, C3-C6heterocycle, aryl, halo, hydroxy, amino, alkoxy and sulfonyl.Additionally, an alkyl group may contain up to 3 heteroatoms. Suitableheteroatoms include nitrogen, oxygen and sulfur.

Aryl groups of the present invention include aryl radicals which mayoptionally contain up to 3 heteroatoms. An aryl group may also beoptionally substituted one or more times with an aryl group or a loweralkyl group. Suitable aryl groups include, for example, phenyl,naphthyl, tolyl, imidazolyl, pyridyl, pyrroyl, thiophenyl, pyrimidyl,thiazolyl and furyl groups.

In one embodiment of the method of this invention, a1,3-diamino-3-substituted-2-propanol chemical intermediate is formedthrough two successive reductions of a nitromethyl amino acid compound,having the structure of structural formula II wherein R⁶ is a carbonylgroup (hereinafter a "3-amino-3-substituted-2-oxo-1-nitropropane").

In the first reduction, a 3-amino-3-substituted-2-oxo-1-nitropropane ismixed, in solution, with a carbonyl reducing agent to form a salt of a1-nitro-3-amino-3-substituted-2-propanol compound, wherein said1-nitro-3-amino-3-substituted-2-propanol compound has the followingstructural formula (structural formula III): ##STR5##

The amount of the carbonyl reducing agent used is an amount which willreduce and hydrogenate at least a portion of the3-amino-3-substituted-2-oxo-1-nitropropane. Typically, from about 0.1moles to about 100 moles of a carbonyl reducing agent are used per moleof 3-amino-3-substituted-2-oxo-1-nitropropane.

A carbonyl reducing agent, suitable for the method of this invention, isa chemical or combination of chemicals which will react with a3-amino-3-substituted-2-oxo-1-nitropropane to reduce and hydrogenate thecarbonyl group, but will generally not affect the nitro group. Suitablecarbonyl reducing agents include, for instance, sodium borohydride,lithium borohydride, borane, disiamylborane,9-bora-bicyclo[3.3.1]nonane, lithium tri-tert-butoxyaluminohydridealuminum hydride, lithium triethylborohydride and lithiumtri(sec-butyl)borohydride.

Suitable solvents for the solution include organic solvents, such asalcohols, esters, ethers and tetrahydrofuran.

It is understood that the 3-amino-3-substituted-2-oxo-1-nitropropane,the carbonyl reducing agent and the solvent may be combinedconcurrently, sequentially, or in any order or combination. It is alsounderstood that the 3-amino-3-substituted-2-oxo-1-nitropropane may beadded as a solid or in solution. It is further understood the carbonylreducing agent may be added as a solid, liquid, in solution or anycombination thereof.

Examples 3 to 6 further describe the reduction of3-amino-3-benzyl-2-oxo-1-nitropropane compounds to1-nitro-3-amino-3-benzyl-2-propanol compounds.

The 3-amino-3-substituted-1-nitro-2-propanol can be either in anoptically pure form, such as a (2R,3S)-diastereomer or a(2S,3S)-diastereomer, or in a racemic mixture. The 2S diastereomer ispreferred. By using chiral reducing agents, the first reduction of3-amino-3-substituted-2-oxo-1-nitropropane can preferentially produceeither the 2S or the 2R diastereomer of3-amino-3-substituted-1-nitro-2-propanol. Chiral reducing agentssuitable for preferentially forming the 2S diastereomer includecombinations of a carbonyl reducing agent, such as lithium aluminumhydride, lithium borohydride or sodium borohydride, with a pureoptically active compound, such as an amino alcohol, sugar orhydroxyalkaloid. Typically, a chiral reducing agent is about 25% toabout 75% (w/w) carbonyl reducing agent and about 25% to about 75% (w/w)optically active compound. A preferred chiral reducing agent for formingthe 2S-diastereomer comprises lithium aluminum hydride and (-) quinine.Other suitable chiral reducing agents include 2,5-dimethylborolane, asdescribed in Imai et al., J. Am. Chem. Soc., 108:7402 (1986),K-glucoride, as described in Brown et al., J. Org. Chem., 53:1231(1988), NB-Enantride, as described in Midland et al., J. Org. Chem.,56:1068 (1991), borane with a chiral oxazaborolidine catalyst, asdescribed in Corey et al., J. Am. Chem. Soc., 109:7925 (1987),R-Alpine-Hydride, obtainable from Aldrich Chemical Co., andS-Alpine-Hydride, also obtainable from Aldrich Chemical Co.

Alternatively, preferential formation of a diastereomer can occurthrough the use of a sterically large (or bulky) carbonyl reducingagent.

To preferentially form a diastereomer, a catalytic amount of a chiralreducing agent is mixed with a 3-amino-3-substituted-1-nitro-2-propanolin an organic solvent and then refluxed at about -10° C. to about 40° C.to form the preferred diastereomer. A catalytic amount is typicallydefined as between about 5% and about 50% (w/w) of the3-amino-3-substituted-1-nitro-2-propanol. Suitable organic solventsinclude alcohols, esters, ethers and tetrahydrofuran.

During the second reduction, the1-nitro-3-amino-3-substituted-2-propanol compound, or salt thereof, isthen mixed in solution with a nitro reducing agent and is therebyreduced to form a 1,3-diamino-3-substituted-2-propanol chemicalintermediate having the structure of structural formula I. Duringreaction, temperature is maintained between about -40° C. and the refluxtemperature of the solvent used. The preferred reaction temperaturerange is from about 20° C. to about 30° C.

In a preferred embodiment, the nitro reducing agent comprises a hydrogensource in the presence of a hydrogenation catalyst. Suitable hydrogensources include, for instance, formic acid, soluble formic acid salts,such as ammonium formate, tetrahydronaphthalene and hydrogen. The amountof the hydrogen source used is an amount which will reduce andhydrogenate at least a portion of the3-amino-3-substituted-2-oxo-1-nitropropane. Typically, the amount of thehydrogen source used is from about 0.1 molar equivalents to about 100molar equivalents per mole of 1-nitro-3-amino-3-substituted-2-propanolcompound.

Hydrogenation catalysts suitable for the second reduction include, forexample, palladium on charcoal, palladium hydroxide, platinum black,platinum oxide, a combination of sodium borohydride and nickel chloride,Raney nickel, or a combination of sodium borohydride and cobaltchloride. The amount of catalyst used is typically from about 0.05 molarequivalents to about 10 molar equivalents per mole of1-nitro-3-amino-3-substituted-2-propanol compound.

Suitable solvents for the solution during the second reduction includeorganic solvents, such as alcohols, alkanes, benzene, ethers, toluene,tetrahydrofuran, or any combination thereof.

To preclude poisoning of the hydrogenation catalyst by a carbonylreducing agent containing boron or sulfur, or wherein the1-nitro-3-amino-3-substituted-2-propanol compound is to be isolated,after the first reduction, the salt of the1-nitro-3-amino-3-substituted-2-propanol compound is acidified with asuitable aqueous acid to form the1-nitro-3-amino-3-substituted-2-propanol compound. Suitable acids arethose acids which will acidify the salt of the1-nitro-3-amino-3-substituted-2-propanol compound, but not cleave theprotecting group. Suitable acids include, for example, KHSO₄, ammoniumchloride and citric acid.

Example 7 further describes the reduction of a1-nitro-3-amino-3-benzyl-2-propanol compound to a1,3-diamino-3-benzyl-2-propanol (or 1,3-diamino-4-phenyl-2-butanol)chemical intermediate.

In another embodiment, the nitro reducing agent, suitable for the methodof this invention, is a chemical or combination of chemicals which willreact to reduce and hydrogenate the nitro group to form an amino group.Suitable second reducing agents include, for instance, lithium aluminumhydride. The amount of the nitro reducing agent used is an amount whichwill reduce and hydrogenate at least a portion of the1-nitro-3-amino-3-substituted-2-propanol compound. Typically, from about0.1 moles to about 100 moles of nitro reducing agent are used per moleof 1-nitro-3-amino-3-substituted-2-propanol compound.

It is understood that the 1-nitro-3-amino-3-substituted-2-propanolcompound, the nitro reducing agent and the solvent may be combinedconcurrently, sequentially, or in any order or combination. It is alsounderstood that the 1-nitro-3-amino-3-substituted-2-propanol compoundmay be added as a solid or in solution. It is further understood thenitro reducing agent may be added as a solid, liquid, gas, slurry,solution or combination thereof.

In an alternate embodiment, 1,3-diamino-3-substituted-2-propanolchemical intermediate is formed via reduction by mixing a3-amino-3-substituted-2-oxo-1-nitropropane with a third reducing agent,wherein said third reducing agent reduces the carbonyl group and thenitro group to form said chemical intermediate. Suitable second reducingagents include, for instance, lithium aluminum hydride.

The amount of the third reducing agent used is an amount which willreduce at least a portion of the3-amino-3-substituted-2-oxo-1-nitropropane to form said chemicalintermediate. Typically, from about 0.1 moles to about 100 moles of athird reducing agent are used per mole of3-amino-3-substituted-2-oxo-1-nitropropane.

It is understood that the 3-amino-3-substituted-2-oxo-1-nitropropane,the third reducing agent and the solvent may be combined concurrently,sequentially, or in any order or combination. It is also understood thatthe 3-amino-3-substituted-2-oxo-1-nitropropane may be added as a solidor in solution. It is further understood the third reducing agent may beadded as a solid, liquid, in solution or any combination thereof.

Example 8 further describes the reduction and hydrogenation of a3-amino-3-benzyl-2-oxo-1-nitropropane compound to form a1,3-diamino-3-benzyl-2-propanol.

A 3-amino-3-substituted-2-oxo-1-nitropropane compound of the presentinvention can be produced from an amino acid represented by thefollowing structural formula (structural formula IV): ##STR6##

In the method for forming a 3-amino-3-substituted-2-oxo-1-nitropropanecompound, said amino acid is mixed with an activating agent and anaprotic solvent under anhydrous conditions to activate said amino acid.An activating agent, as defined herein, is an agent which displaces thehydroxyl of the carboxyl group of the amino acid with a radical suitableto make the carbonyl carbon of said carboxyl group more susceptible tonucleophilic addition. Examples of suitable activating agents include1,1'-carbonyldiimidazole (CDI), isobutyl chloroformate,dimethylaminopropylethylcarbodiimide (EDC), dicyclohexyl carbodiimide(DCC) and N-hydroxysuccimide. For example, wherein CDI is used as anactivating agent, the hydroxyl group of the amino acid is replaced by animidazolyl group.

Suitable aprotic solvents include, for instance, methylene chloride,dimethylformamide, tetrahydrofuran, dichloroethane and diethyl ether.

Anhydrous conditions, as defined herein, means no water is present inthe reagents or solvent and that the reaction is performed in an inertatmosphere, such as under argon or nitrogen. Preferably, no free oxygenis present under anhydrous conditions.

It is understood that the amino acid, the activating agent and thesolvent may be combined concurrently, sequentially, or in any order orcombination. It is also understood that the amino acid may be added as asolid or in solution. It is further understood the activating agent maybe added as a solid, liquid or in solution.

Generally, from about 0.1 moles to about 10 moles of activating agentare used per mole of amino acid. A preferred range is from about 1 moleto about 1.5 moles of activating agent per mole of amino acid.

In one embodiment, the amino acid and activating agent are refluxed todrive the reaction to completion. Typically, refluxing is performed forabout 0.5 hours to about 4 hours, or until gas evolution subsides.

Further description of the formation of activated amino acid isdescribed in Examples 1 and 2.

The activated amino acid is then combined with a nitromethane carbanionsolution under anhydrous conditions to form a reaction mixture, andsubsequently the reaction mixture is acidified to form a3-amino-3-substituted-2-oxo-1-nitropropane compound.

The nitromethane carbanion solution is formed under anhydrous conditionsby mixing an anhydrous base with a nitromethane compound represented bythe structural formula (structural formula V)

    CHR.sup.4 R.sup.5 NO.sub.2

and optionally an aprotic solvent, such as THF. As the formation of thenitromethane carbanion solution is typically exothermic, and as salts ofnitromethane compounds can be unstable and possibly explosive at highertemperatures, the temperature of the nitromethane carbanion solution istypically maintained at a cold temperature, such as about 5° C. or less.

Suitable bases are those which will deprotonate the nirtomethanecompound to form a nitromethane carbanion. Examples of suitableanhydrous bases include metal alkoxides, such as potassium t-butoxideand sodium methoxide, sodium hydride, sodium bicarbonate and lithiumdiisopropylamide. The amount of the anhydrous base used is that amountwhich will deprotonate at least a portion of the nitromethane compoundmolecules to form nitromethane carbanions. Typically, from about 0.1moles to about 1000 moles of anhydrous base are used per mole ofnitromethane compound. It is preferred to use from about 1 mole to about5 moles of anhydrous base per mole of nitromethane compound.

Acids suitable to acidify the reaction mixture consist of acids whichwill reduce pH to a sufficiently low value to prevent significantenolate formation and to react with remaining nitromethane carbanions,but will generally not cleave the protecting group from the3-amino-3-substituted-2-oxo-1-nitropropane. Typically pH is reduced toabout 5 or less, with a pH of 2-5 preferred. Suitable acids include, forinstance, H₂ SO₄, HCl, HBr, H₃ PO₄, KHSO₄, citric acid, acetic acid andcombinations thereof. Wherein the protecting group is a Boc group, acidswhere pH is above 3, such as KHSO₄, are preferred.

Further description of the formation of3-amino-3-benzyl-2-oxo-1-nitropropane is described in Examples 1 and 2.

In a further embodiment, a second chemical intermediate is formed fromthe 1,3-diamino-3-substituted-2-propanol chemical intermediate, whereinthe second chemical intermediate is represented by the followingstructural formula (structural formula VI): ##STR7## and salts thereof.R⁷ is selected from the group consisting of alkyl, hydroxyalkyl,alkoxyalkyl, cycloalkylalkyl, heterocycloalkyl, heterocycloalkylalkyl,aryl, heteroaralkyl, and aminoalkyl radicals. Optionally, an aminoalkylradical may be substituted up to two times with substituents selectedfrom the group consisting of alkyl, cycloalkylalkyl, heterocycloalkyl,heterocycloalkylalkyl, aryl and heteroaralkyl radicals. Furthermore, fora di-substituted aminoalkyl radical, the substituents, combined with thenitrogen atom to which they are bound, may form a heterocycloalkyl or aheteroaryl radical.

To form the second chemical intermediate, a1,3-diamino-3-substituted-2-propanol is mixed with an X¹ -R⁷ compound,where X¹ is a halogen radical, such as chloro or bromo, with a base.Suitable bases include bases which generally will not convert thealcohol group to an alkoxide. Preferably, the base is a mild base, suchas triethylamine. See Example 9 for further description of the synthesisof 1-N-butyl-3-N-Boc-1,3-diamino-3-benzyl-2-propanol by this method.

Compounds, and pharmaceutical compositions, which can be derived fromthe second chemical intermediate include the compounds, andpharmaceutical compositions, described in PCT Patent ApplicationsPCT/US91/08593, by Reed et al., and PCT/US93/04806, by Talley et al.,the teachings of which are incorporated herein by reference.

In another embodiment a third chemical intermediate is formed from a3-N-Boc-1,3-diamino-3-substituted-2-propanol, wherein the third chemicalintermediate is represented by the following structural formula(structural formula VII): ##STR8## and salts thereof. A3-N-Boc-1,3-diamino-3-substituted-2-propanol is oxidized via a Swernoxidation, by mixing the 3-N-Boc-1,3-diamino-3-substituted-2-propanolwith dimethyl sulfoxide and oxalyl chloride, and then adding a base toform said third chemical intermediate. Suitable bases include baseswhich generally will not convert the alcohol group to an alkoxide.Preferably, the base is a mild base, such as triethylamine. Compounds,and pharmaceutical compositions, which can be derived from the thirdchemical intermediate include the compounds, and pharmaceuticalcompositions, described in U.S. Pat. No. 4,692,455, issued to E. M.Gordon, the teachings of which are incorporated herein by reference.

In yet another embodiment, a first anti-hypotensive compound, andpharmaceutical compositions thereof, can also be formed from a3-N-Boc-1,3-diamino-3-substituted-2-propanol compound, wherein theanti-hypotensive compound is represented by the following structuralformula (structural formula VIII): ##STR9## and salts thereof. R⁸ isselected from the group consisting of hydrogen, lower alkyl,halo-substituted lower alkyl, alkaryl, heteroaryl and aminoalkyl. X² isan amino, imino acid or ester radical. Suitable amino, imino acid orester radicals are further described in U.S. Pat. No. 4,604,402, issuedto Godfrey et al., which is incorporated herein by reference.

A first anti-hypotensive compound is formed by mixing a3-N-Boc-1,3-diamino-3-substituted-2-propanol with Cl--C(O)--X₂ in ananhydrous organic aprotic solvent under basic conditions, preferablywith an anhydrous base. Suitable bases include bases which generallywill not convert the alcohol group to an alkoxide. Preferably, the baseis a mild base, such as triethylamine, other alkyl tertiary amines, aryltertiary amines or pyridines. Anti-hypotensive, and pharmaceuticalcompositions, which can be produced according to this method includeanti-hypotensive compounds, and pharmaceutical compositions thereof,described in U.S. Pat. No. 4,604,402.

In an additional embodiment, a second anti-hypotensive compound, orpharmaceutical compositions thereof, can also be formed from theanti-hypotensive compound of structural formula VIII, wherein the secondanti-hypotensive compound is represented by the following structuralformula (structural formula IX): ##STR10## wherein R⁹ is an alkyl oralkaryl. The compound of structural formula IX is then oxidized throughSwern oxidation, through mixing with dimethyl sulfoxide and oxalylchloride, and then adding a weak base, such as triethylamine, to form acompound having the following structural formula (structural formula X):##STR11##

The compound of structural formula X is then contacted with an acid,such as HCl, HBr or H₂ SO₄ to cleave the Boc protecting group and form asalt. Subsequently, the salt is mixed with a base, such astriethylamine, and with a Cl--C(O)--N(R⁹)₂ to form said anti-hypotensivepharmaceutical composition. Anti-hypotensive agents suitable to beformed by this method are further described in U.S. Pat. No. 4,740,508,issued to Weller et al., which is incorporated herein by reference.

Alternatively, said salt comprises a third chemical intermediate, whichcan be used to synthesize ureido-keto and hydroxy-substituted ureidocompounds as described in U.S. Statutory Invention Registration NumberH725, issued to E. M. Gordon.

In another embodiment, a third chemical intermediate is formed from the1,3-diamino-3-substituted-2-propanol chemical intermediate, wherein thethird chemical intermediate is represented by the following structuralformula (structural formula XI): ##STR12## or salts thereof. R¹⁰ isindependently selected from the group consisting of hydrogen, loweralkyl, halo-substituted lower alkyl, aryl and heteroaryl.

In the method for forming the third chemical intermediate, a1,3-diamino-3-substituted-2-propanol is mixed with an aldehyde orketone, having the structural formula R¹⁰ C(O)R¹⁰ (structural formulaXII), in an anhydrous solvent, such as absolute ethanol or toluene, THF,methylene chloride and ethers, to condense the aldehyde/ketone with the1,3-diamino-3-substituted-2-propanol, thereby forming an imino compoundin solution. The imino compound can be isolated but is susceptible tohydrolysis when not retained in solution. The reaction temperature rangeis typically between about 5° C. and about 40° C.

In a preferred embodiment, a condensation catalyst is added to the1,3-diamino-3-substituted-2-propanol and aldehyde/ketone to enhance thecondensation time and yield of imino compound. Suitable condensationcatalysts include, for example, p-toluenesulfonic acid, methanesulfonicand camporsulfonic acid. Typically, the amount of condensation catalystadded is a catalytic amount, for example, about 6.5 to about 10 molarequivalents.

Imino compound solution, within a suitable solvent, is then mixed withan imino reducing agent to cause reductive amination of the iminocompound to form a third chemical intermediate having the structure ofstructural formula XI. The reaction temperature range is typicallybetween about 5° C. and about 35° C. It is preferred that thetemperature of the imino reducing agent be between about 0° C. and about10° C. prior to adding to the imino compound solution in order to makethe reaction less energetic.

Examples of suitable solvents include ethanol, toluene, THF, methylenechloride, ethers, acetic acid or an ethanol/acetic acid mixture.

An imino reducing agent, suitable for the method of this invention, is achemical or combination of chemicals which will reduce and hydrogenatethe imino group. Suitable imino reducing agents include, for instance,sodium borohydride, sodium cyanoborohydride, lithium borohydride andlithium aluminum hydride. The amount of imino reducing agent used is anamount which will cause reductive amination of at least a portion of theimino compound molecules contained in solution. From about 0.1 moles toabout 100 moles, or more, of imino reducing agent can be used per moleof imino compound. Typically, from about 0.5 to about 5 moles of iminoreducing agent are used per mole of imino compound.

In a preferred embodiment, a reductive amination catalyst, such asacetic acid, is also added to the imino compound solution to increasehydrogen generation. Typically the amount of catalyst used is acatalytic amount of about 1-5 wt. %. See Examples 10 and 11 for furtherdescriptions of the formation of third chemical intermediates accordingto this method.

Compounds, and pharmaceutical compositions, which can be derived fromthe third chemical intermediate include compounds and pharmaceuticalscompositions, described in PCT Patent Applications PCT/US91/08593, byRead et al., and PCT/US93/04806, by Talley et al.

The invention will now be further and specifically described by thefollowing examples.

EXAMPLE 1 Synthesis of 3-N-Boc-amino-3-benzyl-2-oxo-1-nitropropane

In an argon atmosphere and under anhydrous conditions, 2.42 moles (391.8g) of 1,1'-carbonyldiimidazole (CDI) and 3 liters of dry THF were mixedin a reactor. 1.89 moles (502.3 g) of Boc-phenylalanine was then addedin five portions to the reactor to form a carbonyldiimidazoleBoc-phenylalanine solution. Vigorous gas evolution was observed from thereaction. The mixture was refluxed for one hour and subsequently cooledto about 30° C.

In a second reactor, 2.42 moles (272 g) of potassium t-butoxide (t-Bu-O⁻K⁺) and 15 L of THF were mixed and then cooled in an ice bath. Dropwise,104 mL (2.46 moles; 159.6 g) of 96% nitromethane was added to theice-cooled t-Bu-O⁻ K⁺ solution to form a pale yellow solution.

The carbonyldiimidazole Boc-phenylalanine solution was then addeddropwise to said pale yellow solution, which was concurrently cooled inan ice bath, to form a reaction mixture. After the addition, thereaction mixture was allowed to stand at room temperature for 12 hoursand then was refluxed for an additional 3 hours to form3-N-Boc-amino-3-benzyl-2-oxo-1-nitropropane in solution in THF.

After refluxing, the product solution was mixed with a 2.5 L aqueoussolution (pH<1) containing 930 g H₂ SO₄ and 530 g KOH to form an organicand an aqueous phase. The organic phase was then concentrated to apaste, while the aqueous phase was then extracted with ethyl acetate.The extracted ethyl acetate and the organic phase's paste were thencombined and subsequently washed twice with aqueous KHSO₄ (final pH ofthe aqueous layer was 3) and then dried over anhydrous MgSO₄, followedby evaporation of the filtered ethyl acetate, to produce yellow, solid3-N-Boc-amino-3-benzyl-2-oxo-1-nitropropane. The crude material wassubsequently used in Examples 3, 4, 5 and 8 without purification.

An analytical specimen of 3-N-Boc-amino-3-benzyl-2-oxo-1-nitropropanewas prepared by recrystallization in ethyl acetate and hexane (2:1) togive a white solid.

¹ H NMR (300 MHz; CDCl₃) shifts observed were 1.40 (s,9H), 3.0-3.1 (m,2H), 4.45 (dd, 1H), 4.9 (bd, 1H), 5.30 (dd, 2H) and 7.2-7.4 (m, 5H). The¹³ C NMR (75 MHz; CDCl₃) shifts observed were 197, 173, 155, 135, 130,128, 83, 82, 60, 37 and 28. Elemental analysis found percents C 58.42, H6.51 and N 9.02 with predicted percents of C 58.42, H 6.54 and N 9.09.Melting point observed was 117°-118° C.

EXAMPLE 2 Synthesis of 3-N-Cbz-amino-3-benzyl-2-oxo-1-nitropropane

111 mmoles (33.4 g) of Cbz-phenylalanine and 133 mmoles (21.6 g) of CDIwere mixed with 600 mL of dry THF in a round bottom flask fitted with areflux condenser. This mixture was then refluxed for 45 minutes to forma yellow solution.

In a second round bottom flask, 133 mmoles (14.9 g) of potassiumt-butoxide, 144 mmoles (9.24 g) of 96% nitromethane and 200 mL of THFwere mixed and cooled in an ice bath for 0.5 hours. The yellow solutionwas then added dropwise via a cannula to the ice-cooled mixture in thesecond round bottom flask to form a reaction mixture. After theaddition, the reaction mixture was allowed to warm to room temperatureand was then refluxed for 17 hours to form the3-N-Cbz-3-amino-3-benzyl-2-oxo-1-nitropropane product in solution. Afterrefluxing, the product solution was brick red and clear. The productsolution was allowed to cool to room temperature.

After cooling, the product solution was then mixed with 250 mL ofsaturated aqueous KHSO₄ solution to acidify the mixture and thenextracted five times with 100 mL aliquots of ethyl acetate. The ethylacetate extracts were dried over anhydrous sodium sulfate. Evaporationof the filtered ethyl acetate produced a paste comprising3-N-Cbz-3-amino-3-benzyl-2-oxo-1-nitropropane.

The 3-N-Cbz-3-amino-3-benzyl-2-oxo-1-nitropropane residue was furtherpurified by recrystallization from ethanol to form an ivory-coloredsolid.

¹ H NMR (300 MHz; CDCl₃) shifts observed were 3.0-3.2 (m, 2H), 4.0-4.5(m, 1H), 5.0-5.4 (m, 4H) and 7.2-7.5 (m, 10H). Melting point range 117°C.-121° C.

EXAMPLE 3 Synthesis of 2R,3S and 2S,3S Diastereomers of3-N-Boc-amino-3-benzyl-1-nitro-2-propanol Using Sodium Borohydride

13.7 mmoles (0.616 g) of 3-N-Boc-amino-3-benzyl-2-oxo-1-nitropropanewere dissolved in 70 mL of methanol and cooled to 0° C. Solid NaBH₄(29.8 mmoles; 1.13 g) was then added to this solution to form a reactionmixture. The reaction mixture was allowed to warm to room temperatureand then stirred for 14 hours. The methanol was evaporated to yield awhite product residue.

The white product residue was then dissolved with 70 mL of water and 70mL of ethyl acetate to form organic and aqueous phases. KHSO₄ (10 g) wasalso added to acidify the aqueous phase. The phases were then separatedby means of a separatory funnel and the aqueous phase was subsequentlyextracted three times with 50 mL aliquots of ethyl acetate. The organicphases were combined, dried over anhydrous sodium sulfate, filtered andthen evaporated to remove the ethyl acetate solvent and produce crude3-N-Boc-amino-3-benzyl-1-nitro-2-propanol.

The crude product was then purified by flash chromatography on a silicagel column using 5:1 hexane/ethyl acetate. The fractions containing thedesired diastereomers were separately pooled and the solvent wasevaporated from each to leave a white residues.

The fraction (R_(f) =0.20) corresponding to the (2R,3S) diastereomer wasthe minor fraction, with a yield of about 13%.

¹ H NMR (300 MHz, CDCl₃) shifts observed were 1.6 (s, 9H), 2.9-3.0 (m,2H), 3.3-3.4 (m, 2H), 3.8-3.9 (m, 1H), 4.3-4.4 (m, 1H), 4.5-4.6 (m, 2H),4.9-5.0 (m, 1H) and 7.2 -7.4 (m, 5H).

The fraction (R_(f) =0.16) corresponding to the (2S,3S) diastereomer wasthe primary fraction, with a yield of about 37%. ¹ H NMR (300 MHz,CDCl₃) shifts observed were 1.3 (s, 9H), 2.8 (dd, 1H), 3.15 (dd, 1H),3.85 (m, 1H), 4.4 (m, 1H), 4.5 (t, 1H), 4.8 (d, 1H), 5.0 (d, 1H), 6.05(bd, 1H) and 7.2-7.3 (m, 5H). The ¹³ C (75 MHz, Acetone-d₆) shiftsobserved were 156, 140, 132, 130, 127, 82, 80, 73, 57, 37 and 28.Elemental analysis found percentages were C 57.91, H 7.18 and N 9.02with predicted percentages of C 58.04, H 7.15 and N 9.03. Melting pointobserved was 144°-144.5° C.

EXAMPLE 4 Synthesis of 3-N-Boc-amino-3-benzyl-1-nitro-2-propanol UsingLithium tri(sec-butyl)borohydride

4.05 mmoles (1.25 g) of 3-N-Boc-amino-3-benzyl-2-oxo-1-nitropropane weredissolved in 100 mL of dry THF and cooled to 0° C. in an ice bath, underargon and with continuous stirring. Six mLs of 1M lithium tri(sec-butyl)borohydride solution in dry THF were then added dropwise to form areaction mixture. The reaction mixture was then maintained at 0° C. for2 hours. During the 2 hour reaction period, a minor amount of gasevolved from the reaction mixture and a light yellow color appeared.After 2 hours, the reaction was quenched by adding about 1 mL of acetonewhile still at 0° C.

The product residue was then mixed with 50 mL of ethyl acetate and 50 mLof 10% aqueous KHSO₄ solution to form organic and aqueous phases and toacidify these phases. The phases were then transferred into a separatoryfunnel with an additional 25 mL of ethyl acetate for rinsing and thenseparated. The aqueous phase was subsequently re-extracted once with a75 mL aliquot of ethyl acetate. The organic phases were combined, washedonce with brine, dried over anhydrous sodium sulfate and then evaporatedby means of a rotary evaporator to form a yellow oil.

The ratio of (2R,3S) and (2S,3S) diastereomers was found to be 1:1 byNMR analysis.

EXAMPLE 5 Synthesis of 3-N-Boc-amino-3-benzyl-1-nitro-2-propanol UsingLithium tri-tert-butoxyaluminohydride

5.51 mmoles (1.70 g) of 3-N-Boc-amino-3-benzyl-2-oxo-1-nitropropane weredissolved in 100 mL of dry THF and cooled to 0° C. in an ice bath, underargon and with continuous stirring. Nine mLs of 1M lithiumtri-tert-butoxyaluminohydride solution in dry THF were then added toform a reaction mixture. A pale yellow color appeared. The reactionmixture was then maintained at 0° C. for 2.15 hours.

The reaction was then quenched by adding 50 mL of 10% aqueous KHSO₄solution while still at 0° C. Upon addition, a minor amount of gasevolved from the reaction mixture and aluminum salts precipitated fromsolution.

The product residue was then transferred to a separatory funnel with 100mL of ethyl acetate to form organic and aqueous phases. The phases wereseparated and then the aqueous phase was subsequently extracted oncewith a 50 mL aliquot of ethyl acetate. The organic phases were combined,washed once with brine, dried over anhydrous sodium sulfate and thenevaporated by means of a rotary evaporator to form a white solid.

The ratio of (2R,3S) and (2S,3S) diastereomers was found to be 1:3 byNMR analysis.

EXAMPLE 6 Synthesis of 3-N-Cbz-amino-3-benzyl-1-nitro-2-propanol

To one gram of 3-N-Cbz-amino-3-benzyl-2-oxo-1-nitropropane, dissolved in30 mL of methanol, was added 0.21 g of NaBH₄ to form a reaction mixture.The reaction mixture was allowed to stand at room temperature and thenstirred for 4 hours. The methanol was evaporated under reduced pressureto yield a product residue.

The product residue was then mixed with 5 mL of aqueous saturatedammonium chloride solution and extracted twice with 30 mL of aliquots ofethyl acetate to form organic and aqueous phases. The phases were thenseparated by means of a separatory funnel. The organic phase was driedover anhydrous magnesium sulfate, filtered and then evaporated to removethe ethyl acetate solvent and produce a fluffy white solid.

The ratio of (2R,3S) and (2S,3S) diastereomers was found to be 18:70 byNMR analysis.

EXAMPLE 7 Synthesis of 3-N-Boc-1,3-diamino-3-benzyl-2-propanol UsingAmmonium Formate

To 14.5 g of 3-N-Boc-amino-3-benzyl-1-nitro-2-propanol, dissolved in 150mL of anhydrous methanol and under argon, was added 1.1 g of Pd/C (5%)catalyst. Ammonium formate (28.1 g) was then added in one portion. Anadditional 250 mL of methanol was subsequently added to facilitatestirring. The mixture was then stirred overnight.

The reaction mixture was then filtered to remove precipitates ofammonium formate. The filtrate was then concentrated to give a whitesolid.

¹ H NMR (300 MHz, Acetone-d₆ DMSO) shifts observed were 1.40 (s, 9H),2.80 (dd, 1H), 2.95-3.05 (m, 3H), 3.15-3.25 (m, 2H), 3.70 (m, 1H), 3.80(m, 1H), 7.20-7.30 (m, 5H) and 8.20 (s, 2H). Melting point observed was134°-136° C.

EXAMPLE 8 Synthesis of 3-N-Boc-1,3-diamino-3-benzyl-2-propanol UsingLithium Aluminum Hydride

2.92 mmoles (1 g) of 3-N-Boc-amino-3-benzyl-2-oxo-1-nitropropane weredissolved in dry THF and cooled to 0° C. in an ice bath, under argon andwith continuous stirring. 11.7 mmoles (0.44 g) of lithium aluminumhydride, dissolved in THF, was added. After gas evolution subsided, themixture was refluxed for 3 hours and then maintained at room temperatureto yield a cloudy solution. Concentrated HCl (4 mL) was then added tomake the solution clear. The solution was then extracted with 200 mL ofmethylene chloride. The combined organic layers were washed with 150 mLof saturated sodium bicarbonate and then dried over magnesium sulfate toyield the product.

EXAMPLE 9 Synthesis of 1-N-butyl-3-N-Boc-1,3-diamino-3-benzyl-2-propanol

1.12 g of 3-N-Boc-1,3-diamino-3-benzyl-2-propanol were dissolved in 7 mLof dimethylformamide. To this solution was added 0.54 g (3.9 mmole)1-bromobutane dropwise. The resulting solution was maintained at 80° C.for 5 hours and then cooled to room temperature before dilution with 50mL of ethyl ether. The resulting solution was washed with two 20 mLaliquots of water. The organic phase was dried (MgSO₄) and concentratedto afford a solid. Column chromatography of the crude product produced1-N-butyl-3-N-Boc-1,3-diamino-3-benzyl-2-propanol.

EXAMPLE 10 Synthesis of(2S,3S)-3-N-Boc-1-N-(4-methoxybenzyl-3-benzyl-2-propanol

3.57 mmoles (0.49 g) of p-anis aldehyde was added to 1.79 mmol (0.50 g)of 3-N-Boc-1,3-diamino-3-benzyl-2-propanol, which was dissolved in 5 mlof ethanol (abs.) to form a reaction mixture. The reaction mixture wasthen stirred at room temperature overnight. Subsequently, 2.65 mmol (100mg) of sodium borohydride, at 0° C., was added to the reaction mixture,in several portions. The reaction mixture was then stirred for another 3hours and allowed to return to room temperature. 5 ml of saturated NH₄Cl solution was then added to the reaction mixture. The resultingmixture was then extracted twice with 20 mL aliquots of ethyl acetate.The ethyl acetate extract was washed with water and brine, then driedover anhydrons sodium sulfate, filtered and then evaporated to removethe solvent and to produce a crude product. The crude product waspurified by flash chromatography on a silica gel column usingmethanol/hexane (1% methanol in hexane). 0.5 g of (2S,3S)-3-N-Boc-1-N-(4-methoxy benzyl).1,3-diamino-3-benzyl-2-propanol wasobtained (70% in yield).

¹ H NMR (300 MHz, CDCl₃) shifts observed were 7.5-7.30 (7H, m),6.85-6.90 (2H, m), 4.80-4.90 (1H, m), 3.85 (3H, s), 3.70-3.80 (3H, m),3.50-3.60 (1H, m), 3.40 (2H, bd), 2.95-3.05 (1H, m), 2.60-2.90 (3H, m),135 (9H, s). Elemental analysis found percentages were C 68.26, H 8.05,N 7.19 with predicted percentages of C 68.94, H 7.92, N 6.99.

EXAMPLE 11 Synthesis of(2S,3S)-3-N-Boc-1N-isobutyl-1,3-diamino-3-benzyl-2-propanol

To 3.57 mmol (1.00 g) 3-N-Boc-1,3-diamino-3-benzyl-2-propanol, which wasdissolved in 25 mL of ethanol (abs.), was added 10.6 mmol (0.77 g) ofisobutyraldehyde, forming a reaction mixture. The mixture was thenstirred overnight at room temperature.

To the above reaction mixture, 0.5 ml acetic acid was added at 0° C.,followed by addition of 4.00 mmol (0.15 g) of sodium borohydzide inportions. Subsequently, the mixture then was stirred for another 6 hoursand allowed to return to room temperature.

Ten (10) ml of saturated NH₄ Cl solution was added to the reactionmixture which then was extracted twice with 20 ml aliquots of ethylacetate. The ethyl acetate extract was washed with water and brine,dried over sodium sulfate, filtered and then evaporated to remove thesolvent thereby producing a white residue. Recrystallization in 15 mlEtoAc 1.5 ml hexane gave 0.97 g of (2S,3S)-3-N-Boc-1-N-isobutyl-1,3-diamino-3-benzyl-2-propanol (80% in yield).

¹ H NMR (300 MHz, CDCl₃) shifts observed were 7.5-7.30 (5H, m), 4.70(1H, m), 3.60-3.70 (1H, bd), 3.45-3.55 (1H, bd), 3.0-3.1 (1H, dd),2.85-2.95 (1H, m), 2.65-2.80 (2H, m), 3.30-2.50(2H, m), 1.75-1.85 (1H,m), 1.35 (9H, s) 1.00 (6H, d). Elemental analysis found percentages wereC 67.28, H 9.38, N 8.13 with predicted percentages of C 67.85, H 9.54, N8.36.

Chemical Analysis

Melting points were determined with a Thomas Hoover capillary meltingpoint apparatus and are uncorrected. Elemental analyses were performedby Atlantic Microlab, Inc., Norcross, Ga. ¹ H NMR spectra were measuredat 300 MHz on a Bruker AC300 and ¹³ C NMR spectra were measured at 75MHz obtained on a Bruker AC300.

Equivalents

Those skilled in the art will recognize, or be able to ascertain, usingno more than routine experimentation, many equivalents to specificembodiments of the invention described specifically herein. Suchequivalents are intended to be encompassed in the scope of the followingclaims.

What is claimed is:
 1. A method for producing a protease inhibitorsynthetic intermediate represented by the formula ##STR13## or saltsthereof, wherein: R¹ is an amino protecting group;R² is selected fromthe group consisting of --H, C1-C18 alkyl, aryl, acetyl and tosyl; R³ isa side-chain of a naturally occurring amino acid or a side chain of anaturally occurring amino acid substituted with a halogen, hydroxyl,sulfonate, C1-C3 alkyl, C1-C3 alkoxy or acyl; R⁴ and R⁵ are eachindependently selected from the group consisting of --H, alkyl, aryl,nitrile and alkoxycarbonyl; and each R⁶ is independently selected fromthe group consisting of hydrogen, lower alkyl, halo-substituted loweralkyl, aryl and heteroaralkyl radicals,comprising the steps of: a)reacting a 1,3-diamino-3-substituted-2-propanol represented by thefollowing structural formula: ##STR14## with an aldehyde or a ketone,having the formula R⁶ C(O)R⁶, in an anhydrous solvent, whereby R⁶ C(O)R⁶is condensed to form an imino compound; and b) reacting said iminocompound with an imino reducing agent, whereby the imino compound isreductively aminated, thereby forming said protease inhibitor syntheticintermediate.
 2. A method of claim 1 further comprising the step ofreacting a condensation catalyst with said1,3-diamino-3-substituted-2-propanol.
 3. A method of claim 2 wherein thecondensation catalyst is selected from the group consisting ofp-toluenesulfonic acid, methanesulfonic acid and camphorsulfonic acid.4. A method of claim 1 further comprising the step of reacting areductive amination catalyst with said imino compound.
 5. A method ofclaim 4 wherein the reductive amination catalyst is acetic acid.
 6. Amethod for producing a protease inhibitor synthetic intermediaterepresented by the formula ##STR15## and salts thereof, wherein: R¹ is aprotecting group;R² is selected from the group consisting of --H, C1-C18alkyl, aryl, heteroaryl, acetyl and tosyl; R³ is a side-chain of anaturally occurring amino acid or a side chain of a naturally occurringamino acid substituted with a halogen, hydroxyl, sulfonate, C1-C3 alkyl,C1-C3 alkoxy or acyl; R⁴ and R⁵ are each independently selected from thegroup consisting of --H, alkyl, aryl, nitrile and alkoxycarbonyl; andeach R¹⁰ is independently selected from the group consisting ofhydrogen, lower alkyl, halo-substituted lower alkyl, alkaryl, arylalkyl,aryl and heteroaralkyl radicals,from a3-amino-3-substituted-2-oxo-1-nitropropane represented by the followingstructural formula: ##STR16## comprising the steps of: a) reacting said3-amino-3-substituted-2-oxo-1-nitropropane with a carbonyl reducingagent, whereby the carbonyl group is reduced to form a1-nitro-3-amino-3-substituted-2-propanol compound represented by thefollowing structural formula: ##STR17## b) reacting said1-nitro-3-amino-3-substituted-2-propanol with a nitro reducing agent,whereby the nitro group is reduced to form a first chemical intermediaterepresented by the following structural formula: ##STR18## c) reactingsaid first chemical intermediate with an aldehyde or a ketone, having aformula R¹⁰ (O)R¹⁰, in an anhydrous solvent, whereby R¹⁰ (O)R¹⁰ iscondensed to form an imino compound; and d) reacting said imino compoundwith an imino reducing agent whereby the the first chemical intermediateis reductively aminated, thereby forming said protease inhibitorsynthetic intermediate.
 7. A method of claim 6 further comprising thestep of reacting a condensation catalyst with said1,3-diamino-3-substituted-2-propanol.
 8. A method of claim 7 wherein thecondensation catalyst is selected from the group consisting ofp-toluenesulfonic acid, methanesulfonic acid and camphorsulfonic acid.9. A method of claim 6 further comprising the step of reacting areductive amination catalyst with said imino compound.
 10. A method ofclaim 9 wherein the reductive amination catalyst is acetic acid.